1. Field of the Invention
This invention relates to a pixel circuit formed using, for example, an organic electroluminescence element, that is, an organic EL element, a display apparatus having a pixel array wherein such pixel circuits are disposed in a matrix, and a driving method for the pixel circuit.
Japanese Patent Laid-Open Nos. 2003-255856 and 2003-271095 are known as related art documents to the inventor.
2. Description of the Related Art
In a display apparatus of the active matrix type wherein an organic electroluminescence (EL) light emitting element is used in a pixel, current to flow through a light emitting element in each pixel circuit is controlled by an active element, usually a thin film transistor (TFT), provided in the pixel circuit. In particular, since an organic EL element is a current light emitting element, a gradation of emitted light is obtained by controlling the amount of current to flow through the EL element.
An example of a related art pixel circuit which uses an organic EL element is shown in FIG. 9A.
It is to be noted that, although only one pixel circuit is shown in FIG. 9A, in an actual display apparatus, m×n such pixel circuits as shown in FIG. 9A are disposed in a matrix, that is, an m×n matrix, such that each pixel circuit is selected and driven by a horizontal selector 101 and a write scanner 102.
Referring to FIG. 9A, the pixel circuit shown includes a sampling transistor Ts in the form of an re-channel TFT, a holding capacitor Cs, a driving transistor Td in the form of a p-channel TFT, and an organic EL element 1. The pixel circuit is disposed at a crossing point between a signal line DTL and a write controlling line WSL. The signal line DTL is connected to a terminal of the sampling transistor Ts and the write controlling line WSL is connected to the gate of the sampling transistor Ts.
The driving transistor Td and the organic EL element 1 are connected in series between a power supply potential Vcc and the ground potential. Further, the sampling transistor Ts and the holding capacitor Cs are connected to the gate of the driving transistor Td. The gate-source voltage of the driving transistor Td is represented by Vgs.
In the pixel circuit, if the write controlling line WSL is placed into a selected state and a signal value corresponding to a luminance signal is applied to the signal line DTL, then the sampling transistor Ts is rendered conducting and the signal value is written into the holding capacitor Cs. The signal potential written in the holding capacitor Cs becomes a gate potential of the driving transistor Td.
If the write controlling line WSL is placed into a non-selected state, then the signal line DTL and the driving transistor Td are electrically disconnected from each other. However, the gate potential of the driving transistor Td is kept stably by the holding capacitor Cs. Then, driving current Ids flows through the driving transistor Td and the organic EL element 1 from the power supply potential Vcc toward the ground potential.
At this time, the current Ids exhibits a value corresponding to the gate-source voltage Vgs of the driving transistor Td, and the organic EL element 1 emits light with a luminance in accordance with the current value.
In particular, in the present pixel circuit, a signal value potential from the signal line DTL is written into the holding capacitor Cs to vary the gate application voltage of the driving transistor Td thereby to control the value of current to flow to the organic EL element 1 to obtain a gradation of color development.
Since the driving transistor Td in the form of a p-channel TFT is connected at the source thereof to the power supply potential Vcc and is designed in such a manner as to normally operate in a saturation region, the driving transistor Td serves as a constant current source having a value given by the following expression (1):Ids=(1/2)·μ·(W/L)·Cox·(Vgs−Vth)2  (1)where Ids is current flowing between the drain and the source of a transistor which operates in a saturation region, μ the mobility, W the channel width, L the channel length, Cox the gate capacitance, and Vth the threshold voltage of the driving transistor Td.
As apparently recognized from the expression (1) above, in the saturation region, the drain current Ids of the transistor is controlled by the gate-source voltage Vgs. Since the gate-source voltage Vgs is kept fixed, the driving transistor Td operates as a constant current source and can drive the organic EL element 1 to emit light with a fixed luminance.
FIG. 9B illustrates a time-dependent variation of the current-voltage (I-V) characteristic of an organic EL element. A curve shown by a solid line indicates a characteristic in an initial state, and another curve shown by a broken line indicates the characteristic after time-dependent variation. Generally, the I-V characteristic of an organic EL element deteriorates as time passes as seen from FIG. 9B. In the pixel circuit of FIG. 9A, the drain voltage of the driving transistor Td varies together with time-dependent variation of the organic. EL element 1. However, since the gate-source voltage Vgs in the pixel circuit of FIG. 9A is fixed, a fixed amount of current flows to the organic EL element 1 and the emitted light luminance does not vary. In short, stabilized gradation control can be carried out.
However, the organic EL element 1 suffers from a drop not only of the driving voltage but also of the light emission efficiency as time passes. In particular, even if the same current flows, the emitted light luminance degrades together with passage of time. As a result, a screen burn occurs that, if a white WINDOW pattern is displayed on the black background and then the white is displayed on the screen as shown, for example, in FIG. 10A, then the luminance at the portion at which the WINDOW pattern is displayed decreases.
In order to compensate for the drop of the light emission efficiency of the organic. EL element 1, such a pixel circuit as shown in FIGS. 11A and 11B has been proposed. Referring to FIG. 11A, the pixel circuit shown includes, in addition to the component of such a pixel circuit as described above with reference to FIG. 9A, a light detection element D1 in the form of, for example, a diode interposed between the gate of the driving transistor Td and the fixed potential.
If the light detection element D1 detects light, then the current therethrough increases. The increasing amount of the current varies in response to the amount of light incident to the light detection element D1. In this instance, the light detection element D1 supplies current in accordance with the amount of emitted light from the organic EL element 1.
For example, when the white is displayed, the light detection element D1 detects emitted light of the organic EL element 1 and supplies current from the fixed power supply to the gate of the driving transistor Td as seen in FIG. 11A. At this time, the gate-source voltage of the driving transistor Td decreases and the current flowing to the organic EL element 1 decreases.
It is assumed that, while the white display is maintained, the emitted light luminance decreases due to a drop of the efficiency of the organic EL element 1 or from some other reason after lapse of a fixed period of time. In this instance, as seen from FIG. 11B, the amount of light incident to the light detection element D1 decreases due to the drop of the emitted light luminance and the value of current flowing from the fixed power supply to the gate of the driving transistor Td decreases. Therefore, the gate-source voltage of the driving transistor Td increases and the current flowing to the organic EL element 1 increases.
As a result, even if the emitted light luminance degrades, the operation of adjusting the amount of current to flow to the organic EL element 1 is carried out by the light detection element D1, and a screen burn arising from a variation of the efficiency of the organic EL element 1 is moderated. For example, the screen burn is reduced as seen in FIG. 10B.